In a TOF-MS, ions accelerated by an electric field are injected into a flight space where no electric field or magnetic field is present. The ions are separated by their mass to charge ratios according to the time of flight until they reach and are detected by a detector. Since the difference in the flight time of two ions having different mass to charge ratios is larger as the flight path is longer, it is preferable to design the flight path as long as possible in order to enhance the resolution of the mass to charge ratio of a TOF-MS. In many cases, however, it is difficult to incorporate a long straight path in a TOF-MS due to the limited overall size, so that various measures have been taken to effectively lengthen the flight length.
In the Japanese Unexamined Patent Publication No. H11-195398, a loop orbit is formed using plural toroidal type sector-formed electric fields, and the ions are guided to fly the loop orbit repeatedly, whereby the effective flight length is elongated. In those mass spectrometers, as the number of turns an ion flies the loop orbit is larger, the flight path is longer, so that the resolution of the mass analysis becomes high as the number of turns is larger.
The mass to charge ratio of an ion is calculated based on the flight time of the ion and the number of turns it flies the loop orbit, where the flight time is the length of time from the time point when the ion starts the ion source and to the time point when it arrives at the ion detector. But, the actual flight time length of an ion is not exactly equal to a calculated one, and the discrepancy between the actual time length and the calculated time length may vary depending on the number of turns. Conventionally such an discrepancy is not taken into consideration in calculating the mass to charge ratio of an ion, so that the value of the calculated mass-to-charge ratio was not adequately reliable and, in the worst case, the calculation results were unable to be used in identifying the sample.
The cause of the discrepancy is not exactly known, but, since the discrepancy appears in a reproducible fashion, it is assumed to be caused as follows. A case is supposed where, as shown in FIG. 3, an ion is introduced into a circular orbit, flies the orbit a predetermined number of turns, and leaves the orbit. The expected orbit is the central one C0 shown by the chain line, and appropriate voltages are applied to the electrodes (not shown in the drawing) arranged along the orbit to produce proper electric fields along the orbit and lets an ion fly the orbit C0. But actually, the electric fields are not produced exactly as designed due to, for example, misshapes in the electrodes or asymmetry of the electrode arrangement, so that an actual orbit deviates from the expected one C0, and an ion may fly a smaller orbit C1 or a larger orbit C2 in a flight. Since the path length of an orbit varies from orbit to orbit, the length of time that an ion flies one turn of an orbit varies even if the speed of the ion is constant. If an ion flies the smaller orbit C1, the flight time is shorter than calculated, and if it flies the larger orbit C2, the flight time is longer than calculated.